The present invention relates to a disk storage device that performs seek control of positioning a head to a target location at a fast speed, and, more specifically, to a disk storage apparatus which performs seek control over a short seek distance of 1 to 10 tracks to handle a sector servo having a relatively long sampling period for sampling a head location signal.
In the prior art disk storage apparatuses such as magnetic disk storage apparatuses, a head is mounted on each data surface of a plurality of disk media. In such a disk storage apparatus, the head switching time is substantially shorter than the time required for traversing one track, namely the one track seek time. To access data that continues beyond one track, a target track address is first sought, and then head switching is performed to access data on a different data surface but in the same track. As track pitch is narrowed with the increase of memory capacity, the offset between heads relatively increases, and thus offset correction is required when head switching is performed. For this reason, the head switching time is going to be equal to or exceed the one track seek time. Furthermore, the relative position of one head to another is likely to change with time, and the seek time is not constant. To write or read continuous data in the prior art, data over different data surfaces but in the same track is written or read, while head switching is performed. In the future, however, it is likely that a faster seek control is achieved by sequentially writing or reading data to or from the same disk surface on a one track seek basis, followed by head switching to shift to the next track.
FIG. 1 shows the change of the head location versus elapsed time in a typical seek control. Seek time, coarse time, settling condition and settling time frequently used in the following discussion are defined as follows. Seek time is the time from the reception of a seek command to positioning the head to its target location with its settling condition met. After the seek, the position error to the target location is measured. The settling condition is the condition that assures position error to within the permissible range of position error. The coarse time that is included in the seek time is the time the head takes to reach the target location from its seek start position, and excludes the settling time. The settling time is the time the head takes to satisfy the settling condition from its arrival to the target location.
I. Position gain
To perform a seek, the current location should be accurately acquired. In the magnetic storage disk apparatus, a location signal, for example, two-phase servo signal, is recorded on a disk medium so that the offset between tracks is determined. This position offset signal is typically an analog signal, and is thus fed to an MPU through an AD converter. To convert the output of the AD converter into a value in the unit of actual track, it should be multiplied by the gain that is beforehand measured on a track by track basis in order to achieve an appropriate position gain.
II. Loop gain
Since the force constant BL of a VCM (Voice Coil Motor) for driving a head actuator usually varies with track position, loop gain should be corrected to make the servo bandwidth of a control system constant. The force constant BL of the VCM and gain of an electric circuit system vary depending on temperature and the like, and correction gain stored beforehand in a firmware becomes unable to match the real value. Since the control system is designed and adjusted such that with no error taking place and its gain corrected, overshoot and undershoot are minimized and the settling time is shortest, variations of gain leads to a prolonged settling time. For this reason, the loop gain should be corrected so that the servo bandwidth of the control system is constant over all tracks.
III. Bias force
External force exerted to the actuator varies from place to place. Correction is required to make an external bias force constant regardless of place.
IV. Head-to-head offset
Head switching is required when the track numbers (cylinder numbers) are the same but with different head numbers. In this case, a head-to-head offset takes place. A one track seek needing head switching is associated with the offset involved in head switching, and thus results in 1.2 track seek in practice, affecting the seek time.
V. Mechanical resonance
Current characteristic with respect to the location of VCM is expressed by an equation that is proportional to a double integral. In practice, however, the current characteristic is influenced by the mechanical resonance between the actuator and the head. Depending on seek current, a mechanical resonance takes place, and a vibration remains after the seek. This post-seek vibration is called residual vibration. If a large residual vibration is generated, a long settling time is required before the vibration ceases.
VI. Runout
A target track on the disk is not concentric but slightly deformed in practice. Among deviation components from an ideal track, a component in synchronism with rotation is called a repeatable runout, and a component not in synchronism with rotation is called non-repeatable runout. The effect of the repeatable and non-repeatable runouts is problematic because of the following two reasons. First, a problem arises from the same track. In the course of the settling determination immediately after a seek, the settling condition remains difficult to satisfy due to the effect of the repeatable and non-repeatable runouts, and thus the settling time is prolonged.
FIGS. 2A and 2B plot the maximum values of each amplitude of the repeatable runout RRO and non-repeatable runout NRRO on each track when a position control system is track following to a target location in a 3.5-inch magnetic disk storage apparatus. As seen from these figures, the repeatable and non-repeatable runout values vary greatly from track to track. A second problem lies in adjacent tracks. To traverse from a current track to an adjacent track, traveling a distance of one track is sufficient enough theoretically. In practice, however, seek distance varies with time and place because of the effect of the repeatable and non-repeatable runouts. FIG. 2C plots the ranges of variations in track pitch measured on each track when one track seek to the adjacent track is performed. In this test, the track pitch varies within a range of .+-.20%.
The disk storage apparatus should perform seek control in optimum fashion to shorten the seek time. To this end, it is necessary to reduce variables such as the position gain, loop gain, bias force, head-to-head offset, mechanical resonance, and repeatable and non-repeatable runouts in disk rotation, as much as possible. The position gain, loop gain, bias force, and head-to-head offset can be measured beforehand, and their effect is canceled by making correction with respect to the current location. Resonance presents no problem in a dedicated servo control apparatus. Specifically, in the dedicated servo control, a head location signal is always obtained from a servo head arranged on a dedicated servo embedding servo information, and a head location is detected in a short sampling period. The effect of strong vibration is avoided by inserting a filter that filters out components on or in the vicinity of a resonance frequency from output currents. In a sector servo control or embedded servo control in which servo information is discretely recorded on tracks of the data surfaces, the sampling period of the head location signal is long. Constructing a filter is physically impossible. There is a possibility that the effect of resonance cannot be excluded. The repeatable runout and non-repeatable runout cannot be fully eliminated, because the tracks are different in shape from track to track. In a short-distance seek control, such as one track seek, precluding the effect of the resonance and repeatable and non-repeatable runouts is vitally important. In the prior art disk storage apparatus using sector servo control having a long sampling period, even a relatively short-distance seek control as short as 1 through 4 tracks needs a long coarse time of 10 samples or longer under the effect of the repeatable and non-repeatable runouts. Hence, a need exists for a faster seek control.